This invention relates to the sealing of beer bottles, and to the compositions for use in this.
A beer bottle is filled with beer and is formed of a bottle body, a cap and a sealing gasket. The body has a neck opening, the cap fits over this opening so as to close it, and the sealing gasket is trapped between the neck opening and the cap.
The gasket must provide a good seal between the body and the cap so as to prevent inward migration of contamination or unwanted outward escape of carbon dioxide. Preventing inward migration is particularly important because beer is very susceptible to the development of off-tastes and these can be caused by a variety of contaminants. For instance the inward migration of oxygen will spoil the flavour as will the inward migration of chlorinated phenols and chlorinated anisoles. Chlorinated phenols are often applied initially as fungicides to wood or other containers in which the beer bottles may be stored, and chlorinated anisoles are often generated as microbial metabolites of the chlorinated phenols.
Unwanted outward migration of carbon dioxide is undesirable since the beer would then acquire a flat taste and texture, and so the seal must withstand moderate pressures, for instance up to about 5 and often about 7 bar without venting. It might be thought that it would be desirable for there to be no sensible upper limit on the pressure that the gasket can withstand without venting. In practice however it is desirable for the gasket to vent at a pressure below a pressure at which the bottle will burst. This is because if a beverage bottle is left in an exposed place, for instance hot sunshine, high pressures can be generated spontaneously. It is desirable that the gasket should vent in preference to the bottle shattering. In practice this means that the gasket should vent before the pressure exceeds around 12 or 13 bar.
The ideal gasket for beer bottles therefore would prevent ingress of oxygen and off flavours and would give a good seal at a moderate internal pressure, typically up to about 5 bar, but would vent at a higher pressure that is below the burst pressure of the bottle, and that is typically in the range 5 to 12 or 13 bar.
The steps of lining the gasket into the cap and of subsquently filling and closing the beer bottles are all conducted at very high speed and so it is necessary that the gasket material should be capable of being used in these high speed processes and that it should give uniform results. For instance it is not satisfactory to use a composition that gives a venting pressure of, for instance, 12 bar in some bottles if it is liable to give venting pressures as low as 9 bar or as high as 15 bar in other bottles since a significant number of the bottles would still be liable to burst and this is unacceptable.
A wide variety of processes and compositions have been proposed for forming the gasket in various container closures, for instance bottle caps. These include plastisols, solutions in organic solvents, aqueous dispersions (including aqueous latices) and mouldable thermoplastic compositions. An early disclosure of the use of thermoplastic compositions for forming container closures is in GB 1,112,023 and 1,112,025. Beer bottles are not mentioned. GB 1,112,023 and 1,112,025 describe a wide variety of ways of introducing the compositions into the cap and a wide variety of thermoplastic compositions that can be used.
Methods that are described in these two patents include inserting and bonding a preformed uniform disc into the cap, inserting and bonding a preformed contoured disc into the cap, flowing a composition into the cap while rotating it and optionally moulding it, flowing a composition into the cap and moulding it while the composition is still hot, inserting a disc of composition carried on a metal plate, transferring composition by a moulding dye and moulding it into the cap, compression moulding the composition into the cap, and so on. In all the examples, the composition was formed into a sheet, discs were cut from it and the discs were then inserted into the caps and cold moulded into the caps. In many of the examples the inserted disc had a diameter substantially the same as the diameter of the cap.
Thermoplastic compositions that were described include blends of ethylene vinyl acetate (EVA) and micro crystalline wax, EVA and low density polyethylene (LDPE) having a melt flow index (MFI) of 7, similar blends containing also butyl rubber having Mooney viscosity of 70, a blend of equal amounts of LDPE having MFI 7 with butyl rubber having Mooney 70, blends of different types of EVA, a blend of LDPE with polyisobutylene, a blend of EVA with ethylene propylene copolymer, an ethylene acrylic acid ester copolymer, a blend of this with LDPE, a blend of LDPE with ethylene propylene copolymer, and a blend of LDPE with chloro sulphonated polyethylene.
Various disclosures of forming gaskets from thermoplastic compositions have appeared from time to time since then and these have listed a wide variety of polymers that can be used. Generally, most of the polymers named above have been listed. An example is EP 331,485 in which molten material is positioned in the cap while still molten (or semi molten) and is moulded into the cap.
In practice, the thermoplastic compositions that have been proposed and used most widely as gaskets for containers are compositions of polyethylenes, ethylene vinyl acetate polymers, and blends thereof. None of the others have attracted any great commercial interest, presumably because of perceived difficulties in making or using the compositions or in their performance.
As indicated, the gasket properties required for beer bottles are quite rigorous. Very good results can be obtained with, for instance, a cap that is a crown closure having a gasket formed of cork lined with aluminium. However this is uneconomic for beer bottle closures and a synthetic polymeric gasket is required.
Of the very wide range of polymeric gasket materials that have been available in recent years, the type that has been used most widely for beer bottles is based on polyvinyl chloride plasticol. However it is well recognised that bottled beer has a relatively short shelf life and can acquire off-tastes on prolonged storage and so a polymeric gasket that permitted a longer shelf life would be highly desirable. Also, the use of polyvinyl chloride in contact with potable or edible materials has in recent years been considered to be undesirable for other reasons and so again it would be desirable to provide beer bottles with an improved type of gasket material.